Microelectronic imagers are used in digital cameras, wireless devices with picture-taking capabilities, and many other applications. Mobile telephones and personal digital assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth rate of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher resolution.
Microelectronic imagers include image sensors that typically may use charged coupled devices (CCDs) and complementary metal-oxide semiconductor (CMOS) devices or other solid state image sensors. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they have low production costs, high yields, small sizes and low power requirements.
In practice, imager modules are fabricated in mass rather than individually. Multiple imager dies, each die including an imager array, are fabricated on an imager wafer. In addition, multiple lens structures, corresponding in number and location to the imager dies on the imager wafer, may be fabricated on a lens wafer. The imager wafer and lens wafer are then assembled with the lens structures being optically aligned with the imager dies to form a plurality of imager modules which may then be separated into individual imager modules. Alternatively, the lens and image sensor can be assembled on die level.
Lens systems for microelectronic imagers should desirably correct for various lens imperfections, such as, for example, chromatic aberration and astigmatism. Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light and results in different wavelengths of light being focused on different positions. Astigmatism is increased with the increasing of thickness of a glass wafer. Astigmatism is a lens aberration in which horizontal and vertical lines are focused at two different points along the optical axis.
Known optical lens systems for correcting chromatic aberration and astigmatism can be complex and costly and difficult to produce particularly at the sizes needed for image sensors. Accordingly, there is a need for an optical lens that is more easily manufactured and which corrects for these lens distortions.